The present invention relates to battery saver type paging receivers and data limiter sections thereof, and more particularly, to an improved peak and valley amplitude detection type data limiter section with mode control operation thereof, digital word storage of acquired peak and valley amplitude values, and self-tuning capabilities.
In a paging system, paging signals are transmitted from a paging transmitter to a multiplicity of portable, battery operated paging receivers according to a prespecified modulation format including serialized digitally coded sync, address, and message data words, for example. Each paging receiver includes an input stage which receives, demodulates, and converts the paging signal into an analog signal having a DC bias level that is dependent on the characteristics of both the receiver circuits and the modulation and a varying AC content which is representative of the serialized digitally coded modulated data of the paging signal.
The next stage of the paging receiver, commonly referred to as the data limiter section, compares the recovered analog signal in time with a threshold signal, which in the ideal case, is intended to be set at the amplitude of the DC content of the analog signal, in order to generate first and second amplitude signals, representative of one and another binary states, for the duration over which the amplitude of the analog signal is above and below the set threshold level, respectively, the serially generated binary states representing bits of the digitally coded words of the paging signal.
Because of the inability to implement ideal super-heterodyne and demodulation functions in the input stage, it was recognized that the DC level could not be considered fixed, but rather relative to the recovered paging signal. Consequently, it was realized that the threshold level of the data limiter could not be simply set at a constant value, but rather had to be dynamically acquired from the immediate paging signal. Formerly, interface circuits employing a coupling capacitor were used to strip off the instantaneous DC content from the recovered analog signal so that the remaining AC content could be compared in time to a fixed level in order to convert the recovered analog signal to a binary bit serial stream of data.
However, these capacitively coupled circuits were found to be incompatible in operation with the battery saving techniques commonly used in the battery operated paging receivers. For a more detailed understanding of the drawbacks of capacitively coupled data limiter circuits, refer to the U.S. Pat. No. 4,631,737; issued Dec. 23, 1986 to Walter L. Davis et al.; and assigned to the same assignee as the present invention. The Davis et al. U.S. Pat. No. 4,631,737 is directed to a direct coupled data limiter circuit which utilizes peak and valley amplitude detectors for dynamically acquiring the instantaneous DC level of the recovered analog signal for use as the threshold level in converting the AC content to its corresponding binary bit serial form. By eliminating the coupling capacitor, the direct coupled circuit of Davis et al. may be rendered operationally compatible with the battery saving cycles of a paging receiver, albeit requiring not insubstantial time to acquire the threshold level.
More specifically, most paging transmission formats include a digitally coded word (sync word) to synchronize the operation of the various paging receivers to the digitally coded address words transmitted successively thereafter in a transmission cycle. Paging receivers customarily are preprogrammed with the slot of the sync word and the slot in which the corresponding address is expected to be transmitted with respect to the sync word. For example, as illustrated in time waveform A of FIG. 1, a sync word may be transmitted in slot 0 of a transmission cycle and the selective page address of our hypothetical paging receiver is preprogrammed to appear at slot 5 with respect thereto. Ideally then, selective circuitry of the paging receiver may be de-energized or put to "sleep" when not needed and energized or "awakened" only during the expected slots of the sync word and corresponding address, which example being illustrated in the time waveform B of FIG. 1.
Battery saving techniques of the paging receiver seek to optimize the control of the battery power supply to selected circuitry thereof by matching the sleep and awake periods as close to the ideal case as practically possible as shown in time waveform B of FIG. 1. One practical limitation to this desired optimization arises as a result of utilizing a peak and valley amplitude detection data limiter circuit in which a substantial amount of time is needed prior to a binary conversion in order to guarantee statistically that at least one peak and at least one valley occurs in the recovered analog signal during the acquisition of the instantaneous threshold level required for the succeeding binary conversion.
Another limitation results from the use capacitors in the data limiter circuit to store the obtained peak and valley signals for a subsequent binary conversion. Since it is a well known phenomenon that a signal stored across a capacitor decays with time due to capacitor leakage, and since the time between binary conversions is not predictable under all circumstances, it was considered prudent practice heretofore to re-acquire new peak and valley signals and corresponding new threshold level immediately preceding each potential sync and address word slot in a transmission cycle. This practice is illustrated by the hatched periods in the time waveform C of FIG. 1. Thus, in most present battery saver type paging receivers, a considerable amount of battery energy is consumed for this purpose rendering a less than optimal battery saving performance thereof.
Accordingly, it is the intent of the present invention to ameliorate the aforementioned limitations of a peak and valley amplitude detection data limiter circuit in order to move closer in operation to ideal battery energy conservation practices to achieve a longer operating life of the paging receiver over the useful life of the battery source thereof.